You can't create it, although the elf at the Winterhold College of Magic does sell it every 42 hours in game. He should be inside the lodging area from morning and at night. He'll sometimes head out to the town bar to get some drinks.
From: Fatso666 | Posted: 11/19/2011 11:37:35 AM cast soul trap on a human and then kill them you need an empty grand soul gem
No, that's not right at all. Grand soul gems do not turn into black soul gems, and grand soul gems cannot hold human souls..
In this game, unlike in Oblivion, you don't MAKE black soul gems. You just acquire them. They can be found in dungeons as well as occasionally in shopkeeper's inventories. --- http://i43.tinypic.com/zydphc.jpg